(PUP-1897) Allow EPP render expression to render a block

If a block is given in <%= {} %> the block should be evaluated, and
what it produces should be rendered. This commit makes this possible
by:
* explicitly handle the render block case
* allowing {} to be validated as r-value producing
Merge pull request #2423 from hlindberg/pup-1897_epp-render-block

(PUP-1897) Allow EPP render expression to render a block